The invention relates to an apparatus and a method for measuring an optical imaging system and to a detector unit which, in particular, can be used for such an apparatus.
Methods and apparatuses of this type are used for determining the imaging quality or image errors of optical imaging systems. The invention relates in particular to methods and apparatuses with which image errors of highly accurate imaging systems, such as are used, for example, in micro-lithography systems for structuring semiconductor devices, can be determined interferometrically with the greatest precision. For this purpose, what is known as the wavefront/image shell measurement by means of shearing interferometry, see for example German laid-open specification DE 101 09 929 A1, and by means of point diffraction interferometry are known as one technique. If, for these interferometric measurements, the same radiation is used as is used by the imaging system in its normal operation, it being possible for the measuring apparatus to be integrated in one structural unit with the imaging system, this is also referred to as an operational interferometer (“Betriebsinterferometer, BIF”). On the other hand, Moiré techniques are usual for distortion measurement.
In the case of the conventional Moiré measuring technique, the entire object field is illuminated and a Moiré object grid structure is introduced into the object plane of the imaging system to be measured and imaged by the latter into the image plane, into which a corresponding Moiré image grid structure is introduced. The Moiré image produced in the image plane is imaged via further imaging optics onto a detector plane, for example of a detector camera. In the case of conventional wavefront measurement by means of shearing interferometry or point diffraction interferometry, on the other hand, a coherence-forming object wavefront-producing structure or a pinhole structure is introduced into the object plane and imaged onto the image plane, in which there is a shearing diffraction grating or a further pin-hole structure. Subsequent imaging optics are arranged in such a way that their object plane lies in the image plane of the imaging system to be measured, so that the exit pupil of the imaging system to be measured is imaged onto the detector plane, for example of a detector camera. On account of these opposite functional principles, hitherto in each case separate measuring apparatuses have been used for wavefront measurement by means of shearing or point diffraction interferometry, on the one hand, and for distortion measurement by means of Moiré methods, on the other hand.